Testing of transimpedance amplifiers

ABSTRACT

Testing is performed on an amplifier wafer housing a transimpedance amplifier prior to packaging the transimpedance amplifier with an external photodetector, wherein the transimpedance amplifier includes a small, auxiliary, integrated silicon photodetector provided at the input of the transimpedance, in parallel with external photodetector attachment points. To test the transimpedance amplifier, the transimpedance amplifier is stimulated by optically exciting the small auxiliary photodetector, wherein the small auxiliary photodetector is excited using short wavelength light, whereby advantages such as higher efficiency may be obtained. The testing method includes placing the amplifier wafer in a testing system, probing the power and ground connections on the amplifier wafer, illuminating the small auxiliary photodetector on the amplifier wafer, and detecting the output of the transimpedance amplifier housed on the amplifier wafer.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application for U.S. patent application Ser. No.10/736,424 filed Dec. 15, 2003.

The present invention relates generally to transimpedance amplifiers,and, more specifically, to semiconductor wafers housing thetransimpedance amplifiers and wafer-level testing the transimpedanceamplifiers prior to packaging the transimpedance amplifiers in opticalreceivers.

BACKGROUND

In an optical communication system, a transimpedance amplifier may beused to amplify an electrical current and convert the electrical currentinto a voltage. A transimpedance amplifier fabricated in silicon CMOS orbipolar technology, for example, may be provided in an optical receiveralong with a photodetector.

FIG. 1 shows an exemplary embodiment of an optical receiver in anoptical link. As shown in FIG. 1, an optical receiver 100 may compriseof a photodetector 300, and a transimpedance amplifier 400 connected tothe photodetector 300 via wirebonds 210.

As shown in FIG. 1, the photodetector 300 receives optical input andconverts the light to a proportional photocurrent. The photocurrent isinput to the transimpedance amplifier 400 via an electrical connection210, where it is converted to a peak-to-peak voltage Vout that isconditioned and amplified.

As a current trend, optical receivers are widely used in optical links.Many optical links operate at wavelengths of 1.3 um or 1.55 um usingsingle mode optical fiber. Optical links may be manufactured at lowercosts using multimode fiber and using a wavelength of 0.85 um. In theseoptical links, the requirements of the photodetectors in the opticalreceiver include, for example:

-   -   high speed, of typically 1-10 Gb/s;    -   high quantum efficiency of greater than 75%;    -   large area of 50-75 um diameter, since the core diameter of        multimode fiber is 50 um or 62.5 um; and    -   low bias voltage of 2-5V.

Given the large absorption length of silicon at a wavelength of 0.85 um,and since silicon is not sensitive to the single-mode fiber wavelengthsof 1.3 um and 1.55 um, it is extremely difficult for a siliconphotodetector to meet all of the requirements for operating in anoptical link. For example, since all CMOS and bipolar processesfabricate the active devices very close to the semiconductor surface,such as within 1.0 um, it is difficult to maintain a high quantumefficiency of greater than 75%.

Accordingly, in optical receivers, photodetectors may be fabricated frommaterials other than silicon. In such cases, the transimpedanceamplifier and the photodetector are fabricated separately, and thenpackaged together with wirebonds or flip-chip attachment afterfabrication in the amplifier circuit.

FIG. 2 shows an exemplary embodiment of a transimpedance amplifier foruse in optical receivers where the transimpedance amplifier and thephotodetector are fabricated separately. As shown in FIG. 2, thetransimpedance amplifier 2000 comprises a substrate 200, and powersupply 500, and ground 600, and amplifier circuit 400 formed on thesubstrate 200. Further, as shown in FIG. 2, a photodetector wiring bondpad 100 for wirebonds or flip-chip attachment to the photodetector afterfabrication is also fabricated on the transimpedance amplifier 2000.

In these embodiments, at least due to a lack of high speed currentsources to drive the input of the transimpedance amplifier, thetransimpedance amplifier 2000 is not tested at high speed until it isdiced and packaged with a photodetector. In addition, because one andtwo dimensional arrays of optical receivers in new packagingtechnologies, such as silicon carrier, may produce individual contactpads as small as 15 um diameter on a pitch of 30 um, and since highspeed probes are typically limited to a minimum pitch of 50-100 um,wafer level probing of the high speed output signals may becomeextremely difficult. Therefore, the transimpedance amplifier 2000 istested after being packaged with a photodetector. Deferring testing tothis stage of fabrication is undesirable since fallout of parts at thisstage may be very costly.

SUMMARY OF THE INVENTION

In accordance with the exemplary aspects of this invention, a testingsystem is provided for testing a transimpedance amplifier separatelyfabricated from a standard III-IV photodetector.

In accordance with the exemplary aspects of this invention, testing isperformed on an amplifier wafer housing the transimpedance amplifierprior to packaging the transimpedance amplifier with an externalphotodetector. In accordance with these exemplary aspects, thetransimpedance amplifier may comprise an amplifier circuit and aphotodetector wirebond pad for attachment to the external photodetector.

In accordance with various exemplary aspects of this invention, a small,auxiliary, integrated silicon photodetector is provided at the input ofthe transimpedance amplifier on the amplifier wafer, in parallel withexternal photodetector attachment points.

In accordance with these exemplary embodiments, the small auxiliaryphotodetector provided on the amplifier wafer does not significantlyaffect the high speed performance of the transimpedance amplifier afterthe transimpedance amplifier is packaged with an external photodetectoron the photodetector wirebond pad.

In accordance with these exemplary embodiments, at least one smallauxiliary photodetector is provided at the input to the transimpedanceamplifier to provide protection from electro-static discharge duringhandling, for example.

In accordance with the exemplary aspects of this invention, the cost oftesting a separately fabricated transimpedance amplifier, which may becostly in the prior art when compared to a monolithic design, may bedecreased. That is, in accordance with these exemplary aspects,advantages such as decrease in cost may be obtained by testing thetransimpedance amplifier at the wafer level, rather than after thetransimpedance amplifier is packaged with an external photodetector.

In accordance with the various exemplary embodiments of the presentinvention, contact-less methods are provided to test the transimpedanceamplifier to verify operation of the transimpedance amplifier.

In accordance with the various exemplary embodiments of the presentinvention, the small auxiliary photodetector is provided to facilitatecontact-less probing at the transimpedance amplifier input.

In accordance with the various exemplary embodiments of the presentinvention, to test the transimpedance amplifier, the transimpedanceamplifier is stimulated by optically exciting the small auxiliaryphotodetector.

In accordance with the various exemplary embodiments of this invention,the small auxiliary photodetector is excited using short wavelengthlight, whereby advantages such as efficiency increase may be obtained.

In accordance with the various exemplary embodiments of the presentinvention, various testing methods are provided, comprising insertingthe amplifier wafer in a testing system, probing the power and groundconnections on the amplifier wafer, illuminating the small auxiliaryphotodetector on the amplifier wafer, and detecting the output of thetransimpedance amplifier housed on the amplifier wafer.

In accordance with an exemplary embodiment of the present invention, theoutput of the transimpedance amplifier is detected by probing the supplyvoltage and detecting the switching currents passing through a bias teeusing a spectrum analyzer.

In accordance with another exemplary embodiment of the presentinvention, the output of the transimpedance amplifier is detected usinga high gain antenna and a sensitive narrow band receiver.

In accordance with an exemplary embodiment of the present invention, theoutput of the transimpedance amplifier is detected using a high speedelectrical probe by either direct contact or capacitive proximitycoupling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary optical receiver.

FIG. 2 shows an exemplary transimpedance amplifier.

FIG. 3 shows an exemplary transimpedance amplifier embodying theexemplary aspects of the present invention.

FIG. 4 shows a top view of an exemplary auxiliary photodetectorembodying the exemplary aspects of the present invention.

FIG. 5 shows a cross sectional view of the auxiliary photodetector ofFIG. 4 along line I-I′.

FIG. 6 shows another exemplary transimpedance amplifier embodying theexemplary aspects of the present invention.

FIG. 7 shows an exemplary testing system embodying the exemplary aspectsof the present invention.

FIG. 8 shows another exemplary testing system embodying the exemplaryaspects of the present invention.

FIG. 9 shows yet another exemplary testing system embodying theexemplary aspects of the present invention.

FIG. 10 shows a flowchart for testing a transimpedance amplifier inaccordance with one exemplary aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description details how exemplary aspects of the presentinvention are employed. Throughout the description of the invention,reference is made to FIGS. 3-10. When referring to the figures, likestructures and elements shown throughout are indicated with likereference numerals.

In FIG. 3, an exemplary transimpedance amplifier embodying the exemplaryaspects of the present invention is shown. As shown in FIG. 3, anamplifier circuit 400 is fabricated on the substrate 200 of thetransimpedance amplifier 3000 along with power supply 500 and ground600. As shown in FIG. 3, the transimpedance amplifier 3000 furthercomprises an on-chip power decoupling capacitor 510 which provides highfrequency currents to the amplifier circuit 400 in addition to the highfrequency currents provided from external sources, and a photodetectorwiring bond pad 100 for wirebonds or flip-chip attachment to an externalphotodetector after fabrication. Further, in accordance with anexemplary embodiment of the present invention, a small auxiliaryphotodetector 700 is provided between the photodetector wirebond pad 100and the power supply 500. As shown in FIG. 3, the small auxiliaryphotodetector 700 is provided at the input of the transimpedanceamplifier in parallel with attachment points to an externalphotodetector.

Though FIG. 3 shows that the decoupling capacitor 510 comprises a singlecapacitor, it should be appreciated this configuration is merely forillustration purposes only. That is, in accordance with the variousexemplary aspects of this invention, the decoupling capacitor mayconsist of a plurality of capacitors, and that this invention is notlimited to the shown single capacitor for providing high frequencycurrents to the amplifier circuit 400.

In various exemplary embodiments, an external photodetector (not shown)is to be packaged with the transimpedance amplifier 3000 afterfabrication by wirebonds or flip-chip attachment on the photodetectorwiring bond pad 100 to form an optical receiver. In these variousexemplary embodiments of the present invention, the small auxiliaryphotodetector 700, though fabricated on the transimpedance amplifier3000 with the amplifier circuit 400, will only be used to test theamplifier wafer and does not affect normal operation of the opticalreceiver. That is, the small auxiliary photodetector 700 does notsignificantly affect the high speed performance of the transimpedanceamplifier 3000 packaged with an external photodetector, and that theprimary function of the small auxiliary photodetector 700 is to providea method for testing the transimpedance amplifier at the wafer level,before it is diced and packaged with the external photodetector.

It is initially noted that the transimpedance amplifier 3000 of FIG. 3is presented for illustration purposes only, and is representative ofcountless configurations in which the exemplary aspects of the presentinvention may be implemented. Thus, the present invention should not beconsidered limited to the system configuration shown in the figure.

For example, it should be appreciated that though the exemplaryembodiment above shows a transimpedance amplifier 3000 used in anoptical receiver, the amplifier circuit may also be used in a number ofsystems known in the art which require an electrical current as aninput. Thus, it is contemplated that the amplifier circuit may beconfigured and may include various topologies and protocols known tothose skilled in the art, while including a small auxiliaryphotodetector provided between the input pad and the input of theamplifier circuit.

In accordance with the various exemplary embodiments of the presentinvention, the substrate 200 of the transimpedance amplifier 3000 may befabricated from materials such as silicon, silicon-on-insulator, galliumarsenide, indium gallium arsenide, and indium phosphide. However, itshould be appreciated that the exemplary aspects of the presentinvention is not limited to only these materials, and that othersubstrates may also be used in this invention.

Further, the amplifier circuit 400 of the transimpedance amplifier 3000may include one of a metal oxide semiconductor circuit, a metalsemiconductor circuit, a bipolar junction transistor circuit, and aheterojunction bipolar transistor circuit. However, it should beappreciated that the exemplary aspects of the present invention is notlimited to only these circuit, and that other amplifier circuits mayalso be used in this invention.

In accordance with the various exemplary embodiments of the presentinvention, external photodetectors to be bonded to the photodetectorbonding pad 100 are fabricated from materials with an absorption regionof III-V semiconductors, such as GaAs or InGaAs, having a performancethat is superior to that of silicon photodetectors at opticalcommunication wavelengths. Furthermore, the requirements of thephotodetectors in the optical links, including high speed (1-40 Gb/s),high quantum efficiency (>75%), large area (50-75 um diameter), and lowbias voltage (2-5V) may be obtained.

It should be appreciated that the small auxiliary photodetector 700 inaccordance with various exemplary embodiments of the present inventionmay be an integrated silicon photodetector fabricated in standard CMOS,CMOS on SOI, or BiCMOS technology. However, it should be appreciatedthat the present invention is not limited to such technology, and mayalso be extended to other semiconductor technologies, such as GaAsMESFET and InP HBT.

In the various exemplary embodiments of the present invention, since thesmall auxiliary photodetector 700 will only be used to test thetransimpedance amplifier 3000, the requirements for large area and highefficiency for a photodetector may be relaxed. Further, the smallauxiliary photodetector may be excited using a source with a shorterwavelength than 0.85 um, which may improve the performance of theamplifier circuit since the absorption coefficient increases at shorterwavelengths.

Thus, by providing the small auxiliary photodetector 700 in thetransimpedance amplifier 3000, during testing, optical packagingrequirements, for example, may be circumvented. For example, optics maybe designed for long working distance as well as small spot focus size,and thus, the long distance permits use of an optical beam splittingcolumn such that existing wafer prober technology may be used.

In accordance with the various exemplary embodiments of the presentinvention, the small auxiliary photodetector 700 may be a P-Nphotodiode, a P-I-N photodiode, a metal-semiconductor-metalphotodetector, or an avalanche photodetector. However, it should beappreciated that the exemplary aspects of the present invention is notlimited to only these photodetectors, and that other photodetectorswhich provides the above described features may also be used in thisinvention.

FIGS. 4 and 5 illustrate an exemplary embodiment of a small auxiliaryphotodetector embodying the exemplary aspects of the present invention.In particular, FIG. 4 shows a sectional view of an auxiliaryphotodetector 700 while FIG. 5 shows a cross view of the small auxiliaryphotodetector of FIG. 4 along line I-I′.

As shown in FIGS. 4 and 5, auxiliary photodetector 700 may be anintegrated silicon photodetector comprising of lateral, interdigitatedp+ fingers 730 and n+ fingers 740, metal pads 720 provided at one end ofthe p+ and n+ fingers 730 and 740, and a p-substrate 710 enclosing theentire auxiliary photodetector 700, wherein an n-well 750 is createdwithin the p-substrate 710 around the p+ fingers 730 and the n+ fingers740. The metal pads 720 serve to connect the photodetector 700 to powersupply 500 and inputs of an amplifier circuit 400 (FIG. 3).

In operation, when the auxiliary photodetector 700 is excited, photonsare absorbed in the lightly doped n-well 750 and are converted toelectrons and holes. A positive bias voltage is applied to the n+fingers 740 with respect to the p+ fingers 730. The electrons and holesdrift due to the electric field and are collected by the n+ fingers 740and p+ fingers 730, respectively. The pn junction formed between then-well 750 and p-substrate 710 then serve to block the slow carriersthat are absorbed deep below the surface of the auxiliary photodetector700.

It is initially noted that the auxiliary photodetector 700 of FIGS. 4-5is presented for illustration purposes only, and is representative ofcountless configurations in which the exemplary aspects of the presentinvention may be implemented. Thus, the present invention should not beconsidered limited to the system configuration shown in the figures.

For example, it should be appreciated that though the exemplaryembodiment above shows the auxiliary photodetector 700 having two p+fingers 730 and two n+ fingers 740, the auxiliary photodetector 700 maycomprise any number of fingers. That is, the auxiliary photodetector 700may be any photodetector in the art for photodetection, provided that itdoes not affect the high speed performance of the amplifier circuit 400.In particular, the auxiliary photodetector 700 is to have lowcapacitance when compared with the capacitance of the III-Vphotodetector.

Further, though FIGS. 4-5 show the auxiliary photodetector 700 in aparticular topology, it is contemplated that the auxiliary photodetector700 may be configured and may include various topologies and protocolsknown to those skilled in the art, while providing the function ofconverting an optical signal into an electrical signal.

Furthermore, though FIG. 3 shows the transimpedance amplifier 3000having a single auxiliary photodetector 700, it is contemplated that aplurality of auxiliary photodetectors may be configured in accordancewith the various aspects of this invention. For example, FIG. 6 showsanother exemplary transimpedance amplifier embodying the exemplaryaspects of the present invention. In accordance with the exemplaryembodiment of FIG. 6, advantages such as on-wafer transimpedanceamplifier testing and electro-static discharge protection may beprovided.

As shown in FIG. 6, transimpedance amplifier 3000′ comprises a firstauxiliary photodetector 700 and a second auxiliary photodetector 800. Inthe transimpedance amplifier 3000′, the first auxiliary photodetector700 is connected between the power supply 500 and the input of theamplifier circuit 400, and the second auxiliary photodetector 800 isconnected between the input of the amplifier circuit 400 and ground 600.

It is to be appreciated that the structure of the exemplarytransimpedance amplifier 3000′ in FIG. 6 is similar to the structure ofthe standard electro-static discharge diodes used in the CMOS process,wherein standard electro-static discharge diodes are often used on abias or signal line to prevent damage to the circuit when a staticcharge is inadvertently applied to the bonding pad. In this exemplaryembodiment of FIG. 6, the first and second auxiliary photodetectors 700and 800 are diodes. Thus, if a large electrostatic charge of positivepolarity appears at the input pad of the transimpedance amplifier 3000′,the first auxiliary photodetector 700 as the top diode will becomeforward biased and will conduct the charge to the power supply 500, thusbypassing and protecting the amplifier circuit 400. Similarly, a largeelectrostatic negative charge will forward bias the second auxiliaryphotodetector 800 as the bottom diode, and the charge will be conductedto ground 600.

In accordance with this exemplary embodiment, with slight modificationto the configuration of the transimpedance amplifier 3000′, theauxiliary photodetectors 700-800 may detect light with the selectiveillumination of the auxiliary photodetectors 700-800 using the steerablefocused beam test configuration.

In accordance with the various exemplary embodiments described above,after the transimpedance amplifier is tested, it may be packaged with astandard III-V photodetector for normal amplifier circuit operation,such as in an optical receiver. Since the auxiliary photodetector issmall, the parasitic capacitance needs to be low enough not tosignificantly degrade the hybrid performance. For example, an auxiliaryphotodetector with an active area of 20 um×20 um would add a parasiticcapacitance of only 20 fF to the amplifier circuit.

Further, it is to be appreciated that this testing method may be appliedto transimpedance amplifiers that are intended for both multimode, i.e.0.85 um, and single-mode, i.e. 1.3 um or 1.55 um, optical links. It isalso to be appreciated that the exemplary testing methods of thisinvention may be applied to both single-ended and differentialtransimpedance amplifier designs.

When testing at wafer level, in addition to the requirement of targetedillumination, high speed probing requirements also require carefulconsideration. Increasing areal density requirements may dictate thatthe individual transimpedance amplifier sizes be decreased into an areaof only 250 um×250 um or less. In new packaging technologies, such assilicon carrier, for example, individual contact pads may be as small as15 um diameter on a pitch of 30 um. These small dimensions are desirablesince it allows more room for the layout of the individualtransimpedance amplifier within the 250 um×250 um amplifier cell.Accordingly, there is a need to provide a method of detecting thefunctionality of the transimpedance amplifier without directlycontacting the high speed outputs of the transimpedance amplifier, sincethe high speed probes are typically limited to a minimum pitch of 50-100um.

In accordance with the various exemplary embodiments of this invention,by providing the auxiliary photodetector to the transimpedance amplifierand using the methods described below, contact-less testing may beperformed to verify operation of the amplifier circuit. The auxiliaryphotodetector may be used to stimulate the input of the amplifiercircuit without directly contacting the amplifier wafer housing thetransimpedance amplifier.

FIGS. 7-9 show various exemplary testing systems in accordance with thevarious aspects of the present invention.

As shown in FIGS. 7-9, a testing system 7000-9000 may be comprised of anillumination system 7200 including microscope column 7220, beamsplitters 7240 and 7250, microscope objective 7225 and illuminationsource 7260, an ancilliary optical system 7350 for collimating laserbeams to be provided to the illumination system 7200, a mechanicaltranslation device 7300 that translates the collimated laser beams inthe X-Y directions, and needle probes 7400 for supplying power andcontrol to an amplifier wafer 3000″ housing the transimpedance amplifierto be tested. As shown in FIGS. 7-9, laser beams f1 and f2 arecollimated by the ancillary optical system 7350 and mounted onmechanical translation stages of the mechanical translation device 7300that permit scanning in the X-Y directions. It should be appreciatedthat a scanning mirror or mirror array may be included to provide anequivalent scanning capability to the laser beams. Laser beams f1 and f2may be modulated at high data rates by modulation sources (not shown).The laser beams f1 and f2 are deflected by the beam splitters 7240 and7250 so that they are focused by the microscope objective 7225 andimpinge on the amplifier wafer 3000″.

It should be appreciated that although two laser beams f1 and f2 areshown in the exemplary embodiments of FIGS. 7-9, other configurationsencompassing a single beam geometry or a multiplicity of beams each ofwhich may be independently steered or gang steered by the mechanicaltranslation device 7300 are possible in accordance with the exemplaryaspects of this invention. It should be appreciated that any number ofsuch independently focused and steered beams may be applied in thevarious aspects of this invention.

In accordance with the various exemplary aspects of this invention,contact-less testing systems 7000-9000 of FIGS. 7-9 are provided toverify operation of a transimpedance amplifier. In these exemplaryembodiments, the laser beams f1 and f2 may be scanned across thetransimpedance amplifier on the amplifier wafer 3000″ without movingother components of the test fixture such as the needle probes and thelike. That is, the laser test beams may be independently positioned andfocused to impinge on the small auxiliary photodetector without movingthe other components. This has advantages for testing throughput sincethe time required to scan the laser beam is much less than the timerequired to mechanically reposition probes 7400.

Though FIGS. 7-9 show that the testing is performed on an amplifierwafer comprising a single transimpedance amplifier, it should beappreciated this configuration is merely for illustration purposes only.That is, in accordance with the various exemplary aspects of thisinvention, the amplifier wafer 3000″ may consist of repeated reticleseach containing a single transimpedance amplifier or an integrated arrayof such transimpedance amplifiers.

In the exemplary embodiments shown in FIGS. 7-9, one of the beams f1 maybe modulated at a first frequency by a first modulation source (notshown), and the other of the beams f2 may be modulated by a secondmodulation source (not shown) at a second frequency which may bedifferent from the first frequency. In the various exemplary aspects ofthis invention, beams having different frequencies while still beingwithin the passband of the transimpedance amplifier may be applied tofacilitate examination of mixing products and cross-talk, for example.In these various exemplary embodiments of testing systems, the output ofthe amplifier wafer 3000″ may be detected using a variety of methods.

FIG. 7 shows an exemplary testing system, wherein the output of theamplifier wafer 3000″ may be indirectly detected. As shown in FIG. 7,testing system 7000 further comprises a bias-tee 7600 to monitor the RFsignature of the activity on a selected power supply of thetransimpedance amplifier on the amplifier wafer 3000″, a probe 7620 forproviding additional power to the amplifier wafer 3000″, and a spectrumanalyzer 7640.

In this exemplary embodiment, a high bandwidth connection may beestablished through the bias-tee 7600 to the probe 7620. The probe 7620provides power to the amplifier wafer 3000″ through the bias tee 7600.This power connection may differ from the power connection provided bythe probes 7400 in that it requires more ripple current when theamplifier circuit is operational. In an exemplary embodiment, the powerconnection is provided to the output drivers of the transimpedanceamplifier on the amplifier wafer 3000″. It should be appreciated that,in a simpler test configuration, the power connections included in theprobes 7400 may be subsumed in the power connection provided by theprobe 7620. However, since the bias tee 7600 is often limited in thetotal amount of DC current that may be provided before compromising theRF choke that is usually present in bias tees, in this exemplaryembodiment, the power is separated into the separate connectionsprovided by the 7400 and 7620 probes.

In the exemplary embodiment of FIG. 7, on the amplifier wafer 3000″, theamplifier circuit 400 (FIG. 3) is stimulated by the ancillary integratedphotodetector 700, and the connections at the power supply 500 andground 600 connecting to the probes 7620 and 7400 may be located onlarge pads at the perimeter of the amplifier wafer 3000″. It should beappreciated that these pads may be the same pads that supply power tothe power supply 500 and/or control in the final product, or may besacrificial pads that may be diced off or left unused e.g. in the caseof a flip chip mounting configuration. That is, the various aspects ofthe present invention are not limited to a particular type of pad.

The bias-tee 7600 then outputs the RF ripple currents to a spectrumanalyzer 7640. During normal operation of the amplifier circuit 400,unbalanced RF currents must be provided by the power supply network. Theunbalanced RF currents may be provided by local on-chip decouplingcapacitors 510 (FIG. 3), with a fraction provided by the external powersupply connection such as through probe 7620. The spectrum analyzer 7640connected to the high frequency signal port of the bias tee 7600 may, bymeans of its large dynamic range, detect even very small RF signaturesemanating from the power connection to probe 7620. The RF signature fromthis power connection has a characteristic harmonic signature or powerspectral density that is unique to the amplifier circuit under test. Theexpected variances in circuit fabrication, bias tee transmission andprobing variability may be accounted for to create a simple pass/failtest. To facilitate this detection and to minimize probing variability,the probe 7620 may be constructed using RF probe techniques that arereadily available.

If the signal reaching the bias tee 7600 is too small to exceed thenoise floor of the spectrum analyzer 7640, test frequency may be loweredto a point that the effectiveness of the on-chip decoupling capacitors510 is no longer sufficient to mask the RF currents needed on the powersupply connection to probe 7620.

FIG. 8 shows another exemplary testing system, wherein the activity ofthe transimpedance amplifier on the amplifier wafer 3000″ may bedetected. As part of normal amplifier operation, RF currents are presenton many of the signal conductors in the amplifier circuit of thetransimpedance amplifier. These RF currents excite electromagneticfields that radiate away from the amplifier wafer 3000″, and may bedetected remotely with a sensitive radio. This radiation efficiency isnormally quite small, but the option exists of selectively patterningsome of the conductor lines in the amplifier circuit to enhance thisradiation efficiency. In this exemplary embodiment, the option exists ofcutting these ancillary structures after testing is completed if theycompromise circuit performance. In the exemplary embodiment shown inFIG. 8, the testing system 8000 further comprises a radio 8200, andprobe 8250.

As shown in FIG. 8, the radio 8200 comprises a sensitive narrow bandreceiver 8220, and a directional high gain antenna 8240. In thisexemplary embodiment, the directional antenna 8240 provides a longworking distance to the amplifier wafer 3000″. Providing a long workingdistance provides the advantage of utilizing conventional wafer probersto test the amplifier wafers since the proximity of the amplifier waferis already occupied by probe cards and optics.

In accordance with the various exemplary aspects of this invention, inan alternative exemplary embodiment, if the radiation efficiency of theconductor traces on the amplifier wafer 3000″ is not sufficient toutilize the directional antenna 8240, proximity capacitive coupling tothe electromagnetic fields excited by the outputs of the transimpedanceamplifier housed on the amplifier wafer 3000″ as depicted by the probe8250 may be used. In this configuration, the probe 8250 is placed inclose proximity of several microns or less above the surface in theproximity of the transimpedance amplifier output. The capacitivecoupling that results from the placement of the probe 8250 maysubstitute for the high gain antenna 8240.

Discrete frequencies may be used to amplitude modulate the laser beamsf1 and f2 with modulation sources. This amplitude modulation of thelaser beams f1 and f2 may be accomplished by direct electricalmodulation of the laser current and is a common technique. The discretefrequencies permit very narrow bandpass filtering on the receiver 8220to increase sensitivity. It should be appreciated that shielding may beprovided to protect the radio 8200 against RF leakage from the opticalcolumn with the modulating laser. The output of the receiver 8220 is fedto a spectrum analyzer 7640 which serves the same function as describedpreviously in conjunction with the power probe 7620 detection schemedepicted in FIG. 7.

FIG. 9 shows yet another exemplary testing system, wherein the outputsof the transimpedance amplifier on the amplifier wafer 3000″ aredirectly probed. As shown in FIG. 9, the testing system 9000 furthercomprises a high speed probe 9220. In this exemplary embodiment, thetesting system 9000 is similar to the testing system 7000 of FIG. 7, butin place of the bias tee 7600 and the probe 7620 of FIG. 7, in theexemplary embodiment of FIG. 9, the power supply to the amplifier wafer3000″ is fed exclusively through the probes 7400. The output of thetransimpedance amplifier housed in the amplifier wafer 3000″ is broughtout to probe-able pads and is contacted by the high speed probe 9220.The output of the high speed probe 9220 may be fed into either aspectrum analyzer or to an oscilloscope. If direct probing is notfeasible, the signal may be coupled to the high speed probe 9220 throughproximity capacitive coupling where the high speed probe 9220 is held inclose proximity of several microns above the transimpedance amplifieroutput but is not required to make contact to the output. In thisexemplary embodiment of FIG. 9, the testing system 9000 still relies onthe auxiliary photodetector 700 to provide an excitation current to theinput of the transimpedance amplifier which is not amenable tosupporting direct electrical probing due to its sensitivity to parasiticcapacitance.

Though FIG. 9 shows that the high speed probe 9220 comprises a singleprobe, it should be appreciated this configuration is merely forillustration purposes only. That is, in accordance with the variousexemplary aspects of this invention, the high speed probe may consist ofa plurality of probes, and that this invention is not limited to theshown single probe for providing output to the spectrum analyzer oroscilloscope.

FIG. 10 shows a flowchart of an exemplary method for testing atransimpedance amplifier according to the various exemplary aspects ofthe present invention. Beginning at step 10000, control proceeds to step11000, where the amplifier wafer housing transimpedance amplifiers isinserted into the testing system. The probes 7400 (and 7620 or 8250 or9220) are positioned over the reticle on the amplifier wafer 3000 thatcontains the next transimpedance amplifier to be tested. In the variousexemplary aspects of the present invention, an auxiliary photodetectoris provided on the amplifier wafer at the input of the transimpedanceamplifier. Next, in step 12000, the power and ground and any requiredcontrol connections on the amplifier wafer are probed. Control thenproceeds to step 13000.

In step 13000, the amplifier wafer is illuminated. That is, focused andmodulated laser beams are impinged upon the auxiliary photodetector onthe amplifier wafer whereby the transimpedance amplifier is stimulatedby the auxiliary photodetector. Next, in step 14000, the output of theamplifier wafer is detected for monitoring the transimpedance amplifierfunctionality. In various exemplary embodiments of this invention, theamplifier wafer is detected using one of a directional high gain antennaand sensitive narrow band receiver, or through a bias tee using aspectrum analyzer. The spectrum analyzer 7640 is then used to make apass/fail decision.

Control then proceed to step 15000 where it is determined if there isanother reticle on the amplifier wafer. If so, control returns to step11000, where the process is to be repeated for testing thetransimpedance amplifier in the next reticle on the amplifier wafer. Ifnot, there are no more reticles on the amplifier wafer and thus thereare no more transimpedance amplifiers to be tested, and controlcontinues to step 16000, where the process ends.

In accordance with the exemplary aspects of this invention, the cost oftesting a separately fabricated transimpedance amplifier, for example,which may be costly in the prior art when compared to a monolithicdesign, may be decreased. This may be most effective when the cost isexacerbated when fabricating arrays of transimpedance amplifiers due tothe decreased yield associated with the multiplicity of circuits.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andother modifications and variations may be possible in light of the aboveteachings. Thus, the embodiments disclosed were chosen and described inorder to best explain the principles of the invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe invention in various embodiments and various modifications as aresuited to the particular use contemplated. It is intended that theappended claims be construed to include other alternative embodiments ofthe invention except insofar as limited by the prior art.

1. A method of testing a transimpedance amplifier at wafer-level,comprising the steps of: inserting a transimpedance amplifier, thetransimpedance amplifier comprising a substrate, an amplifier circuitformed on said substrate, a photodetector pad for connection to anexternal photodetector, and an auxiliary photodetector formed on saidsubstrate adjacent to said amplifier circuit; probing power and groundconnections of said transimpedance amplifier; illuminating saidauxiliary photodetector with modulated laser light deflected by opticalbeam splitters; and detecting output of the transimpedance amplifier. 2.The method according to claim 1, wherein said output is detected using ahigh speed electrical probe by either direct contact or capacitiveproximity coupling.
 3. The method according to claim 1, wherein saidoutput is detected using a directional high gain antenna and a sensitivenarrow band receiver.
 4. The method according to claim 1, wherein saidoutput is detected by probing a supply voltage of the transimpedanceamplifier and detecting switching currents passing through a bias teeusing a spectrum analyzer.
 5. The method according to claim 1, whereinsaid transimpedance amplifier comprises an array of transimpedanceamplifiers.
 6. The method according to claim 5, wherein saidilluminating said auxiliary photodetector further comprises selectivelyilluminating individual auxiliary photodetectors with modulated laserlight deflected by said optical beam splitters.
 7. The method accordingto claim 6, wherein said laser light is steered to selectivelyilluminate said auxiliary photodetectors without moving said power andground connections of said transimpedance amplifiers.
 8. The methodaccording to claim 6, further comprising applying beams having differentfrequencies while being within a passband of the transimpedanceamplifier.
 9. A testing system for testing a transimpedance amplifier atwafer-level, the transimpedance amplifier comprises a substrate, anamplifier circuit formed on said substrate, a photodetector pad forconnection to an external photodetector, and an auxiliary photodetectorformed on said substrate adjacent to said amplifier circuit, the testingsystem comprising: at least one probe for probing power and groundconnections of said transimpedance amplifier; an illumination systemcomprising optical beam splitters for illuminating said auxiliaryphotodetector with modulated laser light deflected by said optical beamsplitters; and a detection device for detecting output of thetransimpedance amplifier.
 10. The system according to claim 9, whereinsaid detection device comprises a high speed electrical probe by eitherdirect contact or capacitive proximity coupling.
 11. The systemaccording to claim 9, wherein said detection device comprises adirectional high gain antenna and a sensitive narrow band receiver. 12.The system according to claim 9, wherein said detection device comprisesa bias tee, and said output is detected by probing a supply voltage ofthe transimpedance amplifier and detecting switching currents passingthrough a bias tee using a spectrum analyzer.
 13. The system accordingto claim 9, wherein said transimpedance amplifier comprises an array oftransimpedance amplifiers.
 14. The system according to claim 13, whereinthe modulated laser light deflected by said optical beam splittersselectively illuminate individual auxiliary photodetectors.
 15. Thesystem according to claim 13, wherein said laser light is steered toselectively illuminate said auxiliary photodetectors without moving saidoptical beam splitters or said probe.
 16. The system according to claim13, wherein said laser light comprises beams having differentfrequencies while being within a passband of the transimpedanceamplifier.